1. Field of the Invention
The present invention relates to a heating system provided with an auxiliary power source which supports an electric power supply to a heating device such as a fixing device, and an image forming apparatus adopting the same.
2. Description of the Related Art
Conventionally, in image forming apparatuses such as a copying machine, a complex machine, a printer and a facsimile machine, there has been a known method of using an auxiliary power source composed of a high-capacity capacitor such as an electric double layer capacitor to support a power supply to a fixing device, as a method for reducing a standby electricity as much as possible and shortening a fixing start-up time i.e. a method for allowing the fixing temperature to rise promptly (for example, refer to Japanese Examined Patent No. 3588006). Since the electric double layer capacitor is capable of charging and discharging electricity at a high current and has a long capacitor life (more than hundred thousand times of charging), it is adapted to be used as an auxiliary power source for a heat generator such as a halogen heater for use in the fixing start-up. However, since a charging voltage of the electric double layer capacitor as a single body is generally about 2.5V, which is low, a method has been devised in which a voltage-raising circuit is provided between the capacitor and the heat generator, and the voltage-raising circuit is used to raise the output voltage (capacitor voltage, discharging voltage) from the capacitor to a necessary voltage for the heat generator (for example, refer to Japanese Patent Unexamined Publication Nos. 2003-297526 and 2004-266984).
In a case where a predetermined voltage is applied to the heat generator with use of the voltage raised by the voltage-raising circuit as described above and outputted from the capacitor, the voltage after the discharging lowers and the input current to the voltage-raising circuit increases when deterioration of the capacitor advances. Accordingly, it is necessary to provide a protective circuit for stopping an output so as not to cause a failure in an input portion of the voltage-raising circuit. However, such method of merely stopping an output in accordance with deterioration of the capacitor causes a problem that the supporting of the electric power supply is disabled as soon as the capacitor reaches the end of a rated life duration (even though use of a capacitor is still possible) even in a case where supporting of an electric power supply by the capacitor should be continued although the fixing start-up time becomes longer than the rated life duration.